In injection molding, molten synthetic materials (such as thermoplastic materials) are passed, for example, through a hot passage manifold system in which there are branches at certain points, into which the molten material supplied in one passage is divided between two discharge passages. These branchings are predominantly of T-shaped configuration.
In the case of a Newtonian liquid flowing through a circular passage, a parabolic flow velocity distribution of the liquid, subdivided into imaginary concentric hollow cylindrical layers sets in, the flow velocity being a maximum in the center of the passage. In such a liquid, the shear between the several imaginary hollow cylindrical layers of the liquid is approximately equal.
On the other hand, a non-Newtonian liquid, such as for example (hot) liquid plastic, behaves differently. In this case, the viscosity is dependent on the shear, which is a maximum near the wall of the circular passage. The less the viscosity, the greater the shear. As a result, the viscosity near the wall of the circular passage is at a minimum. The viscosity distribution of the melt over the cross section resembles a sharply flattened parabola. In a simplified approximate view, this means that in the central region of the passage, the relatively viscous flowing melt behaves like a plug, with a flow velocity approximately independent of the radial location, whereas in the peripheral region the melt is more fluid, owing to the greater shear, and flows more slowly.
This behavior is illustrated in FIGS. 1a-1c. FIG. 1a shows a circular passage through which a non-Newtonian liquid flows, for example a plastic melt. FIG. 1b shows the distribution of the flow velocity “V” over the cross section, and FIG. 1c shows that of the shear. The region “d” corresponds more or less to the aforementioned plug.
If a non-Newtonian liquid flow of the type shown in FIG. 1 is diverted in a rectangular (T-shaped) branching T1 of the passage and divided into two separate flows S1 and S2, as shown in FIG. 2, then the high-viscosity portion and the fluid portion of the liquid will be distributed over the cross section of the passage. The distribution over the cross-section is shown in FIGS. 3a-3c where area HV represents the liquid of high viscosity and the remaining area LV represents the liquid of low viscosity. On the coordinate system drawn in FIGS. 2 to 5, the coordinates x and y lie in the plane of the drawing and the coordinate z runs perpendicular to the plane of the drawing. Thus, the high-viscosity HV portion of the non-Newtonian liquid will collect substantially in the lower portion (in the sense of the drawing) of the passage segments 2a and 2b shown in FIG. 2. This is easily seen, since the viscous fluid (melt) supplied from the central region of the passage segment 1 will advance to the bottom 6 of the Tee, and only then be deflected to the left and right in the sense of FIG. 2, as indicated by the arrows “a” in FIG. 2, while the more fluid liquid flowing in the peripheral region of the passage 1 will be deflected at the very beginning of the branching of the passage, as indicated by the arrows “b”.
If the passage segments 2a and 2b shown in FIG. 2 were very long, then gradually the natural distribution shown in FIG. 3a would gradually be reestablished. In practice, however, the passage segments are short, so that approximately the distribution shown in FIGS. 3b and 3c would be preserved as far as the next deflection in a Tee.
If the liquid flowing in the passage segment 2a encounters the Tee T2, whose lengthwise axis runs in y-direction, the distribution shown in FIG. 4 establishes itself in the discharge passages 3a and 3b. The view here is in flow direction of the discharge passage in question. In the discharge passages, we see a marked inequality of viscous and fluid portions as well as a marked asymmetry of these portions with respect to the centers of the passages.
The Tee T3 in FIG. 2 has two discharge passages 4a and 4b running perpendicular to the plane of the drawing (in z-direction). See FIG. 2a, which shows a top view of this portion of FIG. 2. After deflection in this Tee T2, the separations of viscous and fluid portions of the liquid as shown in FIGS. 5a and 5b result. In the discharge passage 4b emerging upward from the plane of the drawing in FIG. 2, the distribution according to FIG. 5c is established, and in the passage 4b entering the plane of the drawing in FIG. 2, the distribution according to FIG. 5b is established, the view being again defined by the Tee in flow direction of the discharge passage.
In injection molding, if the injection nozzles connected to an injection molding tool (mold) are supplied from passages in which the quantity distribution of melt components of different viscosity is unequal (for example FIGS. 4b and 4c), and/or in which the distribution of the melt is no longer rotationally symmetrical with respect to the longitudinal axis of the passage (for example FIGS. 3b and 5b), this may lead to defects in the cast injection molding products.
If we assume that a plate is injected by way of a plurality of nozzles distributed over the area of the plate, the following defects may occur.
If the portion of the fluid melt from the nozzles in the outer region of the plate is greater than from the nozzles in the inner region of the plate, then under the instant pressure of the entering melt, more melt will be forced into the injection tool (injection mold) in the outer region of the plate than in the middle region. This means that the plate will be supplied with more material per unit area in the outer region than in the inner region, with the result that the cast plate will comprise undular edges. If, conversely, more fluid melt is forced into the injection mold in the inner region, then after cooling of the melt, the greater quantity of melt per unit area in the interior will lead to a bulging of the plate in the inner region.
Similar situations, though less troublesome, arise if the melt portions in the passage segments supplying the nozzle are distributed asymmetrically.
If, for example, each of the several injection nozzles of a hot passage manifold system injects a cup, then the unequal quantity distribution of viscous and fluid melt among various nozzles has the result that the cups will have different wall thicknesses. An asymmetrical distribution of the melt components may lead to that side of the cup which contains preferentially fluid melt becoming thicker than the opposed side of the cup, resulting in a bulged cup, and/or, where viscous melt enters into the mold, it does not get to the bottom of the mold.